The advent of DNA based prenatal diagnosis for human genetic disorders has led to the development of a number of new diagnostic methods. These diagnostic methods permit early detection and consequently informed decisions and intervention with respect to fetus having a genetic disorder. These methods, however, have a number of disadvantages. Each of the new diagnostic methods with which this discussion is concerned requires that a sample of isolated fetal cells be obtained, so that the DNA of the fetus may be examined or tested for signs of specific genetic disorders.
The disadvantages of these modern methods are at least two-fold. First, there is a need to obtain a sample of fetal cells. Currently, fetal cells are obtained by invasive procedures requiring obstetric intervention by amniocentesis or by chorionic villus sampling. These highly specialized procedures carry a small, but significant, risk to the fetus. Early in pregnancy, the level of risk to the fetus is high and the number of cells obtained is low. Therefore, results of these procedures often are not obtained until 18-20 weeks of pregnancy. Second, accurately assessing, quantifying, and assigning a significance to images of cells is difficult, time-consuming and unacceptably subjective.
One modem procedure for obtaining fetal cells relies on leakage of fetal cells into the maternal circulation. By simply drawing a sample of maternal blood, it is theoretically possible to obtain fetal cell material in a sufficient quantity for prenatal diagnosis by DNA based methods. Obtaining fetal cells from the maternal blood circulation avoids any risk to the fetus and can be undertaken as early as 10-12 weeks of pregnancy.
Fetal cells which have been detected in the maternal blood circulation include trophoblasts, lymphocytes and nucleated erythrocytes. Trophoblasts were the first fetal cells to be identified in the maternal blood circulation, due to their large size. However, nucleated erythrocytes have generated the greatest degree of interest as sources of genetic material for prenatal diagnosis due to their rarity in the adult blood circulation, their abundance in fetal blood and their limited life span. These factors combine to reduce errors in distinguishing fetal cell material from maternal cell material. Fetal cells circulating in the maternal blood have a life span ranging from a few weeks (for the nucleated erythrocytes) to a few years (for the lymphocytes).
Although they are consistently present in the maternal blood circulation, fetal cells are very rare, severely limiting their diagnostic utility. Estimates of the concentration of fetal cells within the maternal blood circulation vary widely, from a high level of 1 fetal cell in 105 maternal cells, to a low level of one fetal cell in 109 maternal cells. Thus, a 10 ml sample of maternal blood will ordinarily contain between about 10 and 100 fetal cells. Throughout this description, the concentration of fetal cells found in a freshly drawn maternal blood sample, prior to any further treatment, is referred to as the “naturally present concentration” of fetal cells, typically, but not necessarily, within the above ranges. Also throughout this description, the term “unenriched maternal blood” shall refer to a sample of maternal blood which contains only a naturally present concentration of fetal cells.
Since the naturally present concentration of fetal cells in unenriched maternal blood is so low, in order to obtain a diagnostically significant sample of fetal cells modern techniques include methods of physically isolating the fetal cells from the maternal cells in the sample. In essence, modern techniques are methods of concentrating the fetal cells within a sample, i.e., enriching the sample, for example by removing excess maternal cells, without removing fetal cells. These methods are extremely difficult to perform, often fail to isolate a sufficient number of fetal cells to be diagnostically significant and sometimes fail to provide a sample of a sufficient number of undamaged fetal cells of adequate purity for reliable subsequent diagnosis.
The normal human complement of chromosomes consists of the sex chromosomes (designated X and Y) and 22 autosomes (numbered 1-22). It has been estimated that a minimum of 1 in 10 human conceptions has a chromosome abnormality. As a general rule, an abnormal number of sex chromosomes is not lethal, although infertility can result. In contrast, an abnormal number of autosomes typically results in early death. Of the three autosomal trisomies found in live-born babies (trisomy 21, 18 and 13), only individuals with trisomy 21 (more commonly known as Down's syndrome), survive past infancy.
Although Down's syndrome is easily diagnosed after birth, prenatal diagnosis is problematic. To date, karyotyping of fetal cells remains the established method for the diagnosis of Down's syndrome and other genetic abnormalities associated with an aberration in chromosomal number and/or arrangement. Such genetic abnormalities include, for example, chromosomal additions, deletions, amplifications, translocations and rearrangements. The assessment of such abnormalities is made with respect to the chromosomes of a healthy individual, i.e., an individual having the above-described normal complement of human chromosomes.
Genetic abnormalities include the above-noted trisomies, such as Down's syndrome, as well as monosomies and disomies. Genetic abnormalities also include additions and/or deletions of whole chromosomes and/or chromosome segments. Alterations such as these have been reported to be present in many malignant tumors. Thus, aberrations in chromosome number and/or distribution (e.g., rearrangements, translocations) represent a major cause of mental retardation and malformation syndromes (du Manoir et al., et al., Human Genetics 90 (6): 590-610 (1993)) and possibly, oncogenesis. See also, e.g., (Harrison's Principles of Internal Medicine, 12th edition, ed. Wilson et al., McGraw Hill, N.Y., N.Y., pp. 24-46 (1991)), for a partial list of human genetic diseases that have been mapped to specific chromosomes, and in particular, for a list of X chromosome linked disorders. In view of the growing number of genetic disorders associated with chromosomal aberrations, various attempts have been reported in connection with developing simple, accurate, automated assays for genetic abnormality assessment.
In general, karyotyping is used to diagnose genetic abnormalities that are based upon additions, deletions, amplifications, translocations and rearrangements of an individual's nucleic acid. The “karyotype” refers to the number and structure of the chromosomes of an individual. Typically, the individual's karyotype is obtained by, for example, culturing the individual's peripheral blood lymphocytes until active cell proliferation occurs, preparing single, proliferating (e.g. metaphase, and possibly, interphase) cells for chromosome visualization, fixing the cells to a solid support and subjecting the fixed cells to in situ hybridization to specifically visualize discrete portions of the individual's chromosomes.
The rapid development of non-isotopic in situ hybridization techniques and the general availability of an ever-expanding repertoire of chromosome-specific DNA probes have extended the number of genetic disorders for which karyotyping is feasible. See, e.g., Lichter et al., “Analysis of Genes and Chromosomes by Non-isotopic in situ Hybridization”, GATA 8(1):24-35 (1991). Such methods include the use of probe sets directed to chromosome painting for visualizing one or more preselected chromosomal subregions in a targeted fashion. Methods such as these require at least a modicum of knowledge regarding the types of aberration (s) expected in order to select useful DNA probes complementary to target nucleic acids present in a clinical or tumor cell sample.
Nucleic acid hybridization techniques are based upon the ability of a single stranded oligonucleotide probe to base-pair, i.e., hybridize, with a complementary nucleic acid strand. Exemplary in situ hybridization procedures are disclosed in U.S. Pat. No. 5,225,326 and copending U.S. patent application Ser. No. 07/668,751, the entire contents of which are incorporated herein by reference. Fluorescence in situ hybridization (“FISH”) techniques, in which the nucleic acid probes are labeled with a fluorophor (i.e., a fluorescent tag or label that fluoresces when excited with light of a particular wavelength), represents a powerful tool for the analysis of numerical, as well as structural aberrations chromosomal aberrations. See, e.g., PCT Application WO 94/02646, inventors M. Asgari et al., published Feb. 3, 1994, hereinafter, “Asgari”) co-pending U.S. patent application Ser. No. 07/915,965; P. Lichter, et al., Genet. Anal. Tech. Appl. 8: 24-35 (1991); and S. Du Manoir, et al., Human Genetics 90 (6): 590-610 (1993), the entire contents of which publications are incorporated herein by reference.
Asgari reports in situ hybridization assays for determining the sex of a fetus, genetic characteristics or abnormalities, infectious agents and the like, by nucleic acid hybridization of fetal cells such as those circulating in material blood. The fetal cells are distinguished from maternal cells present in the fixed sample by staining with an antibody which specifically recognizes the maternal or fetal cell or by in situ hybridization to detect one or more fetal mRNAs. The method reportedly is useful for detecting chromosomal abnormalities in fetal cells. However, the fetal cells must be enriched prior to analysis.
PCT Application WO 94/02830, inventors M. Greaves, et al., published Feb. 3, 1994, (hereinafter, “Greaves”) report a method for phenotyping and genotyping a cell sample. The method involves contacting a fixed cell with an antibody labeled with a first fluorophor for phenotyping the cell via histochemical staining, followed by contacting the fixed cell with a DNA probe labeled with a second fluorophor for genotyping the cell. The first and second fluorophors fluoresce at different wavelengths from one another, thereby allowing the phenotypic and genetic analysis on the identical fixed sample.
Despite the above-described advances in the development of fluorescent in situ hybridization methods for the diagnosis of genetic abnormalities, the analysis of the fluorophor-labeled sample remains labor-intensive and involves a significant level of subjectivity. This is particularly true in connection with the prenatal diagnosis of genetic abnormalities in which fetal cells must either be isolated from maternal cells or visually distinguished therefrom prior to assessment for genetic abnormalities. Thus, for example, a laboratory technician must manually prepare and sequentially stain the sample (first, with a histochemical stain to phenotype the cells, second, with a hybridization probe to genotype the cell); visually select fetal cells from other cells in the optical field (using, for example, the above-mentioned histochemical staining procedure); assess the relative distribution of fluorescent color that is attributable to hybridization of the fluorophor-tagged probe; and compare the visually-perceived distribution to that observed in control samples containing a normal human chromosome complement. As will be readily apparent, the above-described procedure is quite time-consuming. Moreover, because the results are visually-perceived, the frequency of erroneous results can vary from one experiment to the next, as well as from one observer to the next.
The discussion thus far has focused on a particular type of rare cell, fetal cells circulating in the mother's peripheral blood, and a particular diagnostic setting, detecting trisomy 21 (Down's syndrome). Many other diagnostic settings are known in which a signal is to be detected in other rare cells. For example, a particular enzyme level or genetic characteristic found in cells of a particular morphology may indicate a condition of medical significance, such as a precancerous condition, a cancerous tumor, a metastasis of a tumor, infection by a virus, and various other genetic conditions, for example.